At present, the virtual reality technology and related applications are developing rapidly. In the virtual reality technology, panoramic images and panoramic videos are an important part. Panoramic video records all surrounding view pictures with a large data amount, so the panoramic video compression is a key technology in virtual reality application. As a new emerging media, compared with the traditional video, panoramic video has the characteristics of wide field of view, high resolution and a large data amount. Using panoramic videos, the observer's view does not change. Change the observation direction to observe all the surrounding scenes. Ordinary two-dimensional video reflects only a part of the panoramic video.
Cylindrical panoramic video is a common panoramic video, equivalent to a virtual camera. The three-dimensional space object is projected onto the cylinder. Cylindrical panoramic videos can be generated by using multi-camera or single-camera acquisition systems.
As the field of view of panoramic video is 5-6 times of that of the normal video. To provide users with the same visual quality, the data volume of panoramic video is 5-6 times of that of the normal video. According to the traditional video transmission scheme, the use of panoramic video in the network environment becomes more and more difficult. Block encoding and transmission have become a common solution for the transmission of panoramic video for users who need to see only part of the panoramic video at the same moment.
Referring to FIG. 1, a transmission method of the cylindrical panoramic video mainly includes the following steps:
A panoramic image is segmented and each image block sequence is encoded independently.
Then the encoded data to be used are selected for transmission. Here one can select the data based on the user's current perspective. The transmission media may be the Internet, wireless networks, local area networks, optical networks, other suitable transmission media, or suitable combinations of these transmission media.
Finally, after the decoder receives the data, these block sequences are independently decoded and transformed to get the desired image.
In the block encoding of panoramic video, the size of the block has important impact on the encoding efficiency of panoramic video and the transmission area. These two factors directly determine the data amount to be transmitted. If the encoding block is small, the transmission area is small, but the encoding efficiency is low. If the encoding block is large, the encoding efficiency is high, but the transmission area is also large. Therefore, for the same visual quality, the data amount to be transmitted for different encoding block sizes is not the same.
In addition, for special characteristics of panoramic videos compared with normal videos, such as circular panoramic videos and high distortion of pictures, a special encoding technology is needed to improve the compression efficiency of the panoramic video.
In the traditional video encoding and decoding standard, an inter-frame prediction method is adopted. The inter-frame prediction method takes an image block as a unit, and selects a block of the same size on the reference image as a prediction block of the current image block. In panoramic video, there is a big distortion in the picture. When the object moves in pictures, zoom-in/zoom-out is produced along with the movement, thus affecting the predictive performance of encoding and the compression efficiency.